Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine in part due to the engine being shut down during times when the engine operates inefficiently, or is not otherwise needed to propel the vehicle. During these conditions, the vehicle is transitioned from an engine mode to an electric mode where the electric motor is used to provide all of the power needed to propel the vehicle. When the driver power demand increases such that the electric motor can no longer provide enough power to meet the demand, or if the battery state of charge (SOC) drops below a certain level, the engine is restarted quickly and smoothly in a manner that is nearly transparent to the driver. Vehicle propulsion is then transitioned from an electric mode to an engine mode.
One method of enabling a smooth engine restart in an HEV powertrain is disclosed by Tulpule et al. in US 20140088805. Therein, a disconnect clutch is disposed between an engine and a motor, which is operable to disconnect the engine from the motor. During an engine restart, the disconnect clutch is disengaged so that the engine can be fueled to obtain a speed that matches the motor speed. Then, when the engine speed matches the motor speed, the disconnect clutch is engaged to couple the engine and the motor to the drive shaft to meet the driver torque demand.
However the inventors herein have recognized potential issues with such an approach. Sufficient disconnect clutch capacity may need to be maintained during the restart to enable the engine to crank through the cylinder compression torque until combustion is initiated. However, depending on the engine position at shutdown, engine piston friction variation, disconnect clutch coefficient of friction variation and other noise factors, it may be challenging to calculate the engine cranking torque required to achieve the desired engine crank and smooth restart performance. If the estimated cranking torque is too low, the engine may not cross through the cylinder compression torque which will cause the engine to rotate first forward and then backward. The resulting crankshaft reversal response can negatively impact the restart response time and lead to NVH issues. In particular, if the engine is rotating backwards at a time when fueling is resumed and the clutch capacity is relatively low, the engine restart may be degraded and the vehicle may stall. In addition to the injected fuel being wasted, additional fuel may be required to restart the engine, degrading fuel economy. Further still, the incomplete engine start may cause a torque disturbance in the vehicle driveline, which in turn can lead to significant NVH issues during the restart.
The inventors have recognized these issues and developed a method for a hybrid vehicle with an improved engine restart method. In one example, the method comprises, during an engine restart, cranking an engine via a motor with a disconnect clutch between the engine and the motor at least partially disengaged while delaying fueling of the engine until only after an engine speed is above a threshold speed and a direction of engine rotation is positive for greater than a threshold duration. In this way, engine fueling may be initiated without incurring crankshaft reversal issues.
In one example, in response to an engine restart request in a hybrid electric vehicle system, the engine may be cranked via an electric motor. While the engine is cranked, a disconnect clutch coupled between the engine and motor may be maintained partially disengaged, with a slippage of the clutch continually adjusted based on a difference between the engine speed and the motor speed. A controller may monitor the engine speed as well as the derivative of the engine speed once the engine starts rotating. Fueling of the engine may be delayed until the controller confirms that the engine speed is higher than a threshold speed as well as the engine rotation is in a positive direction for a threshold amount of time. For example, the controller may compare a preselected number of engine readings to make sure that each consecutive reading is greater than the previous one to confirm that the engine is rotating in the forward direction. Upon confirming that no engine reversals have occurred, engine fueling is resumed. The disconnect clutch is then engaged when the engine speed is within a threshold of the motor speed. In some examples, based on the motor acceleration and the engine acceleration, a motor speed at the desired time of clutch engagement may be predicted, and engine fueling may be adjusted in accordance to expedite transition from the electric mode to the hybrid mode of vehicle operation.
In this way, a quality of engine restarts in a hybrid electric vehicle may be improved. By delaying fueling of an engine following cranking by a motor until the engine speed profile is indicative of positive engine rotation, vehicle stalls can be reduced. By preempting engine fueling during crankshaft reversal, fuel wastage and driveline torque disturbances can be reduced. Overall, a smoother engine restart with reduced NVH issues is enabled, improving operator drive experience.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.